A speaker is a device that generates acoustic signals. A traditional speaker usually includes a moving membrane actuated (e.g. using electromagnetic actuation) by a signal in the Audio frequencies, representing the actual Audio signal that needs to be rendered by a speaker. The moving membrane then creates a local change in air pressure that is related to the audio signal. As a result of these local changes in air pressure an acoustic wave is generated and propagated through the air, thus reproducing the Audio signal used to actuate the membrane. For a given displacement d of a membrane of diameter D, the sound pressure of the wave generated scales with the frequency f as (Ddf)2. (Principles of Vibration and Sound” by Thomas D. Rossing, Neville H. Fletcher, 2nd ed., chapter 10.9 Loudspeakers.) Thus, the Sound Pressure Level (SPL) of such a speaker is decreased with frequency at a rate of 20 dB for drop in frequency by factor 10. Because of such scaling, a traditional speaker requires a large diaphragm and/or large displacement in order to produce low frequency sounds. This fundamental principle is a limitation on the design of small sized speakers, used in mobile devices, for example.
One design of small speakers for use in mobile devices is known as a micro-electromechanical system pico-speaker or “MEMS pico-speaker” and is described in the U.S. Pat. No. 8,861,752. The same scaling of sound pressure increasing with frequency to a power of two is used in the pico-speaker. As described in U.S. Pat. No. 8,861,752, a membrane is oscillated at an ultrasonic frequency that is modulated with the wanted audio signal. An acoustic shutter is then used to obstruct and open the air flow of the ultrasonic wave generated by the oscillating membrane, thus modulating this ultrasonic wave. Operating the shutter/modulator at ultrasonic frequency related to the central frequency used for the membrane can result in generating output air pressure in audio frequencies range, corresponding to the wanted audio signal.
Some embodiments of the present disclosure are generally related to improvements in the acoustic modulator used in a MEMS pico speaker, but the same embodiments may be used for other purposes, wherever acoustic modulation can be of use.